1. Field of the Invention
The invention relates to septic tanks generally, and more particularly to a water-tight septic tank.
2. Discussion of the Background
Tanks for the first phase treatment of home waste have traditionally been referred to as septic tanks. Septic tanks slow down and temporarily hold a waste flow from a home so that a natural cleansing action involving anaerobic bacteria can occur. As sewage enters the tank, solids separate from fluids and fall to the bottom. Anaerobic bacteria break the solids down to reduce the volume. However, the volume of the solids is never reduced to zero; therefore, a residue remains. It is this residue that must be removed from the septic tank.
An example of a typical septic tank system 100 is shown in FIG. 1. The system 100 includes a tank 110 with an inlet 120 and an outlet 130. The tank 100 also includes an access riser 140. As sewage enters the tank 110, it separates into one of three distinct layers in the tank. The top layer 170 is referred to as scum. This layer includes components of the sewage such as cooking oils and fats, cigarette filters, and other materials that float. Most of the scum does not decompose in the septic tank 110. The middle layer 160 is referred to as the clear layer. The scum layer 170 actually floats on the middle layer 160 such that approximately three quarters of the volume of the scum layer 170 is below the liquid level 161 of the middle layer 160 and approximately one quarter of the volume of the scum layer 170 is above the liquid level 161. The middle layer 160 comprises mostly liquids with suspended solids. Bacteria digest many of the suspended solids in this area. The bottom layer 150 is referred to as sludge. This layer contains most of the sewage solids and is generally oxygen free. Anaerobic bacteria reduce the volume of solids in this layer greatly, but not entirely.
In operation, raw sewage enters the inlet 120. The inlet 120 is typically shaped as a "T" or "Y" to prevent clogging of the inlet 120 by the scum layer 170. The raw sewage separates into the three layers 150, 160, 170 as discussed above, where the bacteria begin breaking the sewage down. As sewage enters the tank 110 through the inlet 120, a corresponding volume of sewage from the clear layer 160 exits the tank 110 through the outlet 130, which is also "T" or "Y" shaped for the same reason the inlet is shaped that way. As long as the fluid in the clear area 160 has been present in the tank for a sufficient amount of time for the bacteria to do their work, the fluid exiting the tank through the outlet 130 is relatively clear. The exiting fluid is typically released into the surrounding soil, where further biodegradation continues.
However, if the fluid in the clear layer 160 has not been in the tank 110 for a sufficient amount of time, solids are released with the fluid through the outlet 130 and into the surrounding soil, causing the surrounding soil to become clogged or blocked. At this point the system 100 has failed. One main cause of septic system 100 failure is the entry of ground water into the tank 110, which causes the premature release of fluid in the layer 160 from the tank 110. Therefore, it is important that the tank 110 be watertight.
The use of baffles has become popular in recent years. A baffle 190 is shown in phantom in FIG. 1. The baffle 190 is essentially a vertical wall that separates the tank 100 into two portions, usually such that the first portion (the portion nearest the inlet 120) is at least 2/3 of the volume of the tank. The baffle 190 usually includes a passage located in the clear layer 160. The idea is to allow clear fluids only past the baffle so that further biodegradation can occur before the fluids are expelled through the outlet 130. Because only fluids pass through the baffle 190, the size of the sludge and scum layers 150, 170 is theoretically greatly reduced relative to the clear layer 160 in the portion of the tank 110 downstream of the baffle 190. There is ongoing dispute within the industry as to the effectiveness of baffles.
Prior art tanks are often made of concrete or rotocast thermoplastic. The problem with concrete tanks is that historically they have been fabricated poorly. Also, concrete leaks unless it is professionally designed and manufactured and then carefully installed by a trained and experienced field installer. When those precautions are taken, the cost of the tank is increased and becomes non-competitive in many instances. A leaking septic tank interrupts the treatment process as described above. On the other hand, thermoplastic tanks are often watertight, but are not strong. Over time, they collapse under loads when being driven over by trucks or under other hydrostatic loads. Another weakness is that such rotocast thermoplastic tanks will often collapse when pumped dry during the periodic sludge removal process.
Another popular method for handling home sewage is to pipe the sewage from the home to a central treatment plant. This plant is usually built by a municipality. The waste that is pumped to the plant is not treated before it leaves the home and is therefore thicker because it includes solids. Transporting waste in this manner requires large pipes made of concrete. Such pipes are usually six inches or larger in diameter. In addition, getting the waste to the plant requires either expensive pumping stations or installing the pipes with the correct pitch for a gravity flow system, which can add significantly to the expense in installing the pipes at the requisite depths.
An additional problem is that the market is changing. Some municipalities are no longer accepting new subscribers. Developers are often now required to install their own treatment systems when they build development sites.